A digital front end (DFE) is employed in communication devices to perform signal processing on transmitted and received signals. In the transmitter, for example, the DFE comprises the signal processing components that process the baseband signal in the digital domain and provide an up-converted digital signal to a digital-to-analog converter (DAC). The DFE in the transmitter typically includes, for example, components to perform RF equalization and I/Q imbalance correction.
Generally, the RF equalizer compensates for dispersion in the RF path (e.g., in PCB board traces and RF cables) and/or for the frequency dependent linear response of a power amplifier and other RF circuits in the transmitter chain. When a channel has been properly equalized, the frequency domain attributes of the signal at the input are reproduced at the output.
In a Quadrature Amplitude Modulation (QAM) communication system, the in-phase (“I”) and quadrature (“Q”) components of a signal identify a symbol being communicated. The IQ ratio is typically modulated by controlling the amplitudes of two sinusoids separated in phase by ninety degrees. An IQ imbalance is introduced when the two generated sinusoids are not perfectly matched in amplitude and orthogonal in phase, causing a received point to be mis-aligned with a constellation point corresponding to the desired symbol. A number of techniques have been proposed or suggested for correcting IQ imbalance. For example, U.S. Pat. Nos. 7.167,513 and 7,336,730 are directed to techniques for correcting IQ imbalance.
A digital front end is typically implemented using hardwired logic due to the high sampling rates. IQ imbalance correction and RF equalization, for example, are typically performed using two separate hardware modules. While such hardware-based DFE techniques effectively process a communication signal, they suffer from a number of limitations, which if overcome, could further improve the efficiency and flexibility of DFE systems. For example, existing hardware-based DFE techniques lack flexibility and it is expensive, time consuming and challenging to modify the DFE design for a new RF design.
A need exists for software implementations of IQ imbalance correction and RF equalization.